maint: Use parentheses around condition for switch(),while(),if() statements. * libinterp/corefcn/dirfns.cc, libinterp/octave-value/ov-fcn-handle.cc, liboctave/array/Sparse.cc, scripts/image/rgb2ind.m, scripts/io/importdata.m, scripts/io/strread.m, scripts/optimization/fminbnd.m, scripts/optimization/sqp.m, scripts/plot/graphics_toolkit.m, scripts/plot/hdl2struct.m, scripts/plot/legend.m, scripts/plot/print.m, scripts/plot/printd.m, scripts/plot/private/__contour__.m, scripts/plot/private/__go_draw_axes__.m, scripts/plot/struct2hdl.m, scripts/polynomial/polyeig.m, scripts/sparse/bicg.m, scripts/specfun/ellipke.m, scripts/special-matrix/gallery.m, scripts/ui/errordlg.m, scripts/ui/helpdlg.m, scripts/ui/inputdlg.m, scripts/ui/listdlg.m, scripts/ui/questdlg.m, scripts/ui/warndlg.m: Use parentheses around condition for switch(),while(),if() statements.

--- a/scripts/plot/graphics_toolkit.m
+++ b/scripts/plot/graphics_toolkit.m
@@ -101,7 +101,7 @@
 
 %!testif HAVE_FLTK
 %! old_toolkit = graphics_toolkit ();
-%! switch old_toolkit
+%! switch (old_toolkit)
 %!   case {"gnuplot"}
 %!     new_toolkit = "fltk";
 %!   otherwise